Servoactuator having motor-driven actuator with hydraulic force amplification

ABSTRACT

A servoactuator ( 20 ) is operatively arranged to control the movement of an output member ( 21 ) in either of two directions in response to a command signal. The servoactuator includes an electric motor ( 25 ); a motor controller ( 24 ); a first transmission mechanism ( 34 ); a hydrostatic second transmission mechanism ( 35 ); a transfer mechanism ( 36 ) operatively arranged to selectively couple the motor output shaft to the output member either through the first transmission mechanism to impart a high-speed low-force drive to the output member, or through the second transmission mechanism to impart a low-speed high-force drive to the output member; at least one feed-back transducer ( 29, 32 ); and a servo control loop ( 30, 33 ) closed about the motor, controller, transmission mechanisms, transfer mechanism, feedback transducer and output member for selectively controlling at least one of the position, velocity or force of the output member as a function of the command signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/884,626, filed Jun. 19, 2001, now abandoned, which is acontinuation of prior application Ser. No. 09/708,904, filed Nov. 8,2000, now abandoned.

TECHNICAL FIELD

The present invention relates generally to the field of actuators formoving a load, and, more particularly, to an improved two-stagemotor-driven actuator with either a ball-screw or hydraulic first-stageand with a hydraulic second-stage, which is particularly adapted for usein selectively opening and closing a mold used in a plastic moldingmachine.

BACKGROUND ART

A plastic molding machine typically has two mold halves that must beselectively opened and closed during an operational cycle. After apreceding article has been formed and removed, one mold half must bemoved rapidly toward its cooperative mate at relatively low force toinitially close the mold. Thereafter, high force is required through ashort stroke to clamp the mold in a closed position before plasticmaterial is injected therein.

To meet these requirements with electric motor drives requires either anexcessively large motor to provide the high force needed to close themold, or a non-linear toggle mechanism to approximately match theperformance of a smaller motor to the load requirements.

Recent improvements avoid these alternatives by effectively introducinga type of selectable “gear shift” in the mold-closing drive. This “gearshift” operates uni-directionally (i.e., only as the mold is beingclosed and clamped, but not as the mold is being unclamped an opened)through a direct-acting motor-driven ball-screw to rapidly advance onemold half toward the other, followed by a hydrostatic force amplifier orintensifier in which the motor-driven screw moves a small piston thathydraulically communicates with a large piston (which was been bypassedduring the rapid mold movement) to develop the high force necessary toclamp the mold.

U.S. Pat. No. 4,968,239 (facially assigned to Fanuc Ltd.) discloses anelectric motor drive for a plastic molding machine. This drive providesa type of “manual transmission” by providing different ratio gear trainswhich can be alternately selected by means of electromagnetic clutches.However, this reference does not teach the use of a hydrostatictransmission to selectively couple the large-stroke low-force gear trainto the short-stroke high-force gear train.

U.S. Pat. No. 5,345,766 (facially assigned to Engel Maschinenbau GmbH)discloses a motor/ball-screw drive for a plastic molding machine inwhich the motor drives through a single-sided piston which is springloaded to a fixed position on the machine platen. When the driving forcerequirements exceed the spring loading, the piston is moved relative tothe platen to displace fluid into a parallel single-sided larger-areapiston-and-cylinder arrangement to produce a hydrostatically-amplifiedclamping force. Alternative valving means are described to allow thefilling of the large cylinder as the platen is advanced. However, thisreference does not appear to teach or suggest that the operation may bereversed when it is desired to open the mold.

U.S. Pat. No. 6,439,875 B1 (facially assigned to Kabushiki Kaisya MeikiSeisakusyo) also discloses a motor-driven ball-screw arrangement inwhich the motor drives through a single-sided piston which is springloaded to a fixed position. When the driving force requirements exceedthe spring loading, the piston is moved relative to the platen todisplace fluid into a parallel single-sided larger-areapiston-and-cylinder arrangement to produce a hydrostatically-amplifiedclamping force. However, the small intensifier piston may be blocked bya solenoid valve, and the fluid supplied to the clamping piston is alsocontrolled by a solenoid valve. Additionally, the large clamping pistonhas a short stroke that may be selectively coupled to the moving memberat any position. Here again, this reference does not appear to teach orsuggest that the operation may be reversed when it its desired to openthe mold.

Recent improvements in injection molding processes include the techniqueof bringing the mold halves to an almost-closed position, injectingmolten plastic, and then driving the mold closed to compress theplastic. This process requires that the mold actuator be moved preciselyto a predetermined position and held there as plastic is injected,followed by advancing to the fully-closed position against the highcompression load. The actuator must, therefore, be capable ofclosed-loop servo position control in both the low- and high-forcemodes. Closed-loop control, in turn, necessitates that the actuator becontrollable in both directions.

Additionally, after an article has been molded by this method, thebreakaway mold opening force that is needed to initially crack the moldmay be substantially greater than the force for which the directmotor/screw drive would preferably be designed for normal rapid motionof the mold platen, thus requiring intensified force in the reversedirection to break open the mold. This is followed by a long-strokelow-force movement of one mold half relative to the other to fully openthe mold.

Accordingly, it would be generally desirable to provide improvedapparatus for moving one mold half relative to another, which apparatusaffords the capability of a high-speed low-force approach as one moldhalf moves toward the other, including holding a predetermined positionunder closed-loop servo control, followed by a low-speed high-forceclamping as the two mold halves are clamped together, and a low-speedhigh-force breakaway drive as the two mold halves are initiallyunclamped followed by a high-speed low-force movement of one mold halfaway from the other.

DISCLOSURE OF THE INVENTION

With parenthetical reference to the corresponding parts, portions orsurfaces of the embodiment shown in FIG. 1, merely for purposes ofillustration and not by way of limitation, the present invention broadlyprovides an improved servoactuator (20) for selectively controlling themovement of an output member (21) in either of two opposite directionsin response to a command signal.

The improved servoactuator broadly includes: an electric motor (25)having a rotatable output shaft (26); a motor controller (24) arrangedto provide a drive current(s) to the motor for controllably rotating theoutput shaft; a first transmission mechanism (34) operatively arrangedto displace the output member in either direction at a nominal firstratio with respect to the motor output shaft; a hydrostatic secondtransmission mechanism (35) operatively arranged to displace the outputmember in either direction at nominal second ratio with respect to themotor output shaft; a transfer mechanism (36) operatively arranged toselectively couple the motor output shaft to the output member eitherthrough the first transmission mechanism to impart a high-speedlow-force drive to the output member, or, alternatively, through thesecond transmission mechanism to impart a low-speed high-force drive tothe output member; at least one feedback transducer (29, 32) capable ofmeasuring one of the force, displacement or velocity of the outputmember; and a servo control loop (30, 33) closed about the motor,controller, transmission mechanisms, transfer mechanism, feedbacktransducer and output member for selectively controlling at least one ofthe position, velocity or force of the output member as a function ofthe command signal. This is traditionally done by having a summing pointin the servoloop provide an error signal as the algebraic sum of acommand signal and a negative feedback signal.

The invention may further include an engaging device (e.g., 61 in FIG.2) for operatively coupling the second transmission mechanism to theoutput member at any position of the output member.

The first transmission mechanism may be hydromechanical or hydrostatic.A plurality of valve components (51, 53, 55), which may be eitherphysically separated from one another or mounted in a common body, maybe operatively arranged to couple the first and second transmissionmechanisms hydraulically when the output member is to be displaced atthe nominal second ratio with respect to the motor output shaft. Thefirst transmission mechanism may include a screw thread (28) and nut(39). The transfer mechanism may include a plurality of valves (51, 53,55) operatively arranged to selectively either (a) lock the nut to theoutput member and allow the second transmission mechanism to bedecoupled from the output member, or (b) couple motion of the nutrelative to the output member through the second transmission mechanismto the output member.

Accordingly, the general object of the invention is to provide animproved servoactuator for selectively controlling the movement of anoutput member (e.g., a movable mold half) in either of two directions asa function of a supplied command signal.

Another object is to provide an improved servoactuator for use inplastic molding machinery, which allows one mold half to initiallyadvance toward another by means of a high-speed low-force drive, andthereafter allows the two mold halves to be clamped together by means ofa low-speed high-force drive.

Still another object is to provide an improved servoactuator for use inplastic molding machinery that allows two mold halves to be separated bymeans of a low-speed high-force drive to initially unclamp the moldhalves, followed by a high-speed low-force drive to quickly move theseparated mold halves away from one another.

These and other objects and advantages will become apparent from theforegoing and ongoing written specification, the drawings, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first form of the improvedservoactuator, this view showing the output member as being movablerelative to a stationary or fixed member, showing the motor-driven firsttransmission mechanism and the hydrostatic second transmission mechanismselectively coupled by means of a transfer mechanism including threediscrete solenoid valves, and also showing the inclusion of feedbackforce and displacement/velocity transducers.

FIG. 2 is a schematic view of the second form of the improvedservoactuator, generally similar to FIG. 1, but showing the inclusion ofan engaging device for selectively coupling the second transmissionmechanism to the stationary member at any position of the output member.

FIG. 3 is a schematic view of a third form of the improvedservoactuator, this view showing hydrostatic first and secondtransmission mechanisms, the inclusion of a motor-driven pump, and twofeedback transducers supplying their respective signals to the motorcontroller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., cross-hatching, arrangement of parts, proportion,degree, etc.) together with the specification, and are to be considereda portion of the entire written description of this invention. As usedin the following description, the terms “horizontal”, “vertical”,“left”, “right”, “up” and “down”, as well as adjectival and adverbialderivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”,etc.), simply refer to the orientation of the illustrated structure asthe particular drawing figure faces the reader. Similarly, the terms“inwardly” and “outwardly” generally refer to the orientation of asurface relative to its axis of elongation, or axis of rotation, asappropriate.

First Embodiment (FIG. 1)

Referring now to the drawings, and, more particularly, to FIG. 1thereof, a first form of the improved servoactuator is generallyindicated at 20.

This servoactuator is operatively arranged to selectively control themovement of an output member, generally indicated at 21, in eitherhorizontal direction (i.e., leftwardly or rightwardly) relative to astationary support 22 in response to a command signal supplied on line23 to a motor controller, indicated by box 24. Controller 24 isoperatively arranged to provide appropriate drive current(s) via line 27to an electric motor, indicated in box 25, having a rotatable outputshaft 26. Shaft 26 is shown as having an externally-threaded portion 28.

A feedback transducer 29 is operatively arranged to sense either theposition or velocity, as appropriate, of the output member 21 relativeto stationary member 22, and to provide such information as a feedbacksignal via line 30 to controller 24.

Output member 21 is also shown as having a clevis member 31 connected toan opposing load, such as a mold half (not shown) of a plastic moldingmachine (not shown). A force transducer 32 is operatively interposedbetween the output member 21 and clevis member 31, and is operativelyarranged to supply a force feedback signal via line 33 to motorcontroller 24.

The servoactuator is shown as having a first transmission mechanism 34operatively arranged to displace the output member in either horizontaldirection at a nominal first ratio with respect to the rotation of themotor output shaft. The servo-actuator is also shown as having ahydrostatic second transmission mechanism, generally indicated at 35,that is operatively arranged to displace the output member in eitherhorizontal direction at a nominal second ratio with respect to therotation of the motor output shaft. The output member is further shownas having a transfer mechanism, collectively indicated at 36, that isoperatively arranged between the first and second transmissionmechanisms for selectively coupling the motor output shaft to the outputmember either through the first transmission mechanism to impart ahigh-speed low-force drive to the output member, or through the secondtransmission mechanism to impart a low-speed high-force drive to theoutput member.

More particularly, the first transmission mechanism is shown as having ahorizontally-elongated specially-configured tubular member 38. A portionof member 38 is internally threaded, as indicated at 39, to form anut-like member, and these threads are in mating engagement with theexternal threads 28 on output shaft 26. An annular flange extendsradially outwardly from an intermediate portion of member 38 to form apiston 40 which is mounted for sealed sliding movement within a cylinder41 provided in the output member. Piston 40 thus sealingly subdividescylinder 41 into a left chamber 42 and a right chamber 43. Piston 40 hasequal annular areas facing into chambers 42 and 43.

The second transmission mechanism 35 is more fully shown as including ahorizontally-elongated rod 44 having its left end mounted on stationaryportion 22 and as extending rightwardly therefrom. A piston 45 is fixedto rod 44, and is mounted for sealed sliding movement within a cylinder46 provided in the output member. Piston 45 has equal-area annularsurfaces facing into left and right chambers 48, 49, respectively.

Chambers 48 and 49 communicate with one another via a line 50 whichcontains a two-position solenoid valve 51. First transmission mechanismleft chamber 42 communicates with line 50 via line 52, which alsocontains a two-position solenoid valve 53. First transmission mechanismright chamber 43 communicates with line 50 via line 54, which containsyet another two-position solenoid valve 55. Solenoid valves 51, 53, 55are controlled by suitable means that are well known to persons skilledin this art, but have been deliberately omitted from FIG. 1 to avoidobfuscating the invention.

In operation, clevis member 31 is connected to an opposing load (e.g., amovable mold half). A command signal is provided via line 23 to motorcontroller 24. This causes the controller to send appropriate drivecurrent(s) via line 27 to electric motor 25, which selectively rotatesoutput shaft 26 in the appropriate angular direction.

Solenoids 53 and 55 are normally closed to trap fluid in chambers 42,43. Since hydraulic fluid is relatively incompressible, rotation ofshaft 26 causes a corresponding horizontal movement, either leftwardlyor rightwardly depending upon the direction of rotation of shaft 26, ofthe output member 21 by virtue of the screw thread connection betweenshaft 26 and threaded portion 39. This then causes horizontal movementof the output member in either direction at a nominal first ratio withrespect to the rotation of the motor output shaft. At the same time,solenoid 51 is open so as to allow fluid to flow freely between secondtransmission mechanism chambers 48 and 49. Thus, the first transmissionmechanism may be used to quickly move the output member relative to thestationary portion at relatively low force.

The force exerted by the load on the output member is sensed by forcesensor 32, which provides a feedback signal via line 33 to controller24. Similarly, the position (i.e., displacement or velocity, asappropriate) of the movable member 21 relative to the stationary member22 is continuously monitored by means of transducer 29, which, in turn,provides a feedback signal via line 30 to controller 24.

When the controller (or some other controlling device) senses that theload has increased beyond a predetermined value, as during mold closing,appropriate signals are sent to the three solenoid valves to closesolenoid valve 51 and to open solenoid valves 53 and 55. Thereafter,continued rotation of the motor output shaft will cause movement offirst transmission mechanism piston 40 within cylinder 41. Since chamber42 communicates with chamber 48 via now-opened solenoid valve 53, andsince chamber 43 communicates with chamber 49 via now-opened solenoidvalve 55, continued rotation of the motor output shaft will cause somerelative movement between piston 40 and cylinder 41, creatingcommunicating flows with respect to these chambers. This then creates aforce amplification, which is a function of the ratio of the areas ofthe annular faces of pistons 45 and 41, to continue movement of theoutput member in the indicated direction at a lower speed, but at asubstantially increased force.

Thus, if the load is a mold half, the first transmission mechanism maybe used to provide a high-speed low-force drive to the output member asthe mold halves are moved toward one another, and the secondtransmission mechanism may be thereafter used to create a low-speedhigh-force drive, as when the two mold halves are being clampedtogether. When it is desired to open the mold halves, the operation isreversed, with the second transmission mechanism creating a low-speedhigh-force drive to the output member to initially break the two moldhalves apart, followed by use of the first transmission mechanism toquickly move the now-unclamped mold halves rapidly away from oneanother.

Second Embodiment (FIG. 2)

A second form of the improved servoactuator is generally indicated at 60in FIG. 2. This second form contains many of the parts and componentsthat were contained in the first embodiment. Hence, wherever possible,the same reference numeral has been used in FIG. 2 to indicate likeparts and components previously-described. Because of this, a detaileddescription of these previously-described parts will be omitted, withthe following description focusing not on the similarities, but thedifferences between the second embodiment shown in FIG. 2 with respectto the first embodiment shown in FIG. 1.

While output member 21 is depicted as being physically somewhatdifferent, it will be noted that it has, in substance, the samefunctional elements previously described.

The salient difference between FIGS. 2 and 1 is that the secondembodiment shown in FIG. 2 contains an engaging device, generallyindicated at 61, that is operatively arranged within stationary member22. This engaging device has a piston-like member 62 that contains adownwardly-extending lug or boss 63. Rod 44, rather than being fixed tostationary support 22, is now mounted for selective movement through ahole provided in the stationary member. The left marginal end portion ofrod 44 is of slightly-enlarged radius, and has a plurality ofaxially-spaced annular grooves or recesses, severally indicated at 64,that are configured and arranged to accommodate insertion of boss or lug63. As indicated by the arrows 65, member 62 is adapted to beselectively moved toward and away from rod 44 to lock lug 63 into analigned one of recesses 64, and away from rod 44 to release thisconnection. The means or mechanism for moving member 62 are notspecifically shown. It might, for example, be a hydraulic arrangement,or the like. The specific implementation is considered to be well withinthe ability of a person skilled in this art, but has been omitted in theinterest of clarity. The command for such movement is shown as beingsupplied from controller 24 or some other controller.

The second embodiment operates substantially as the first embodiment,except as indicated below. When the first transmission mechanism is usedto quickly move the output member and load, controller 24 causes member62 to move upwardly within the recess provided in stationary part 22 toallow free horizontal sliding movement of rod 44 within the openingprovided through stationary member 22. During this mode, all threesolenoid valves 51, 53 and 55 are closed so that both pistons, 40 and45, are hydraulically locked in their respective cylinders. However,when the controller senses that the pressure is building, as when themold is beginning to clamp, the controller sends an appropriate signalto move member 62 downwardly so that lug 63 snaps into one of recesses64. At the same time, the controller supplies appropriate signals toopen solenoid valves 53 and 55, as previous described. Thereafter,continued operation of the motor will cause fluid flow from the firsttransmission mechanism to the second transmission mechanism to provide alow-speed high-force drive to the output member, as when the mold halvesare being clamped or unclamped.

Operation in the opposite direction is the reverse of thatpreviously-described, with the rapid movement of the output member beingcontrolled by the first transmission mechanism.

Third Embodiment (FIG. 3)

A third form of the improved servoactuator is generally indicated at 70in FIG. 3. Here again, this third form contains many of the portions orelements previously described. Hence, the same reference numeral will beused to describe the same structure, except where indicated. This thirdform has an output member 21 that is mounted for horizontal movement,either leftwardly or rightwardly, relative to stationary member 22.

However, piston 40 is mounted on a rod 71 having its left end fixed tostationary member 22 and extending rightwardly therefrom. Similarly, thesecond transmission mechanism includes rod 44 having its left end fixedto stationary member 22. In this form, the motor 25 has an output shaft72 that is arranged to rotate the impeller of a pump 73 to displacefluid between first transmission mechanism chambers 42 and 43,respectively. In other words, the output shaft 26 having the screwthread 28 in FIG. 1 has been replaced by an output shaft 72 and pump 73.Thus, when the load is less than the predetermined minimum load,controller 24 will transmit appropriate drive current(s) to motor 25 tocause rotation of output shaft 72 and operation of pump 73. This willpump fluid between first transmission mechanism chambers 42 and 43 tomove the output member, either leftwardly or rightwardly (asappropriate), relative to rod 71 and stationary member 22. As with thefirst embodiment, when this occurs, the solenoid valves 53 and 55 areclosed, while solenoid valve 51 is opened to allow fluid communicationbetween second transmission mechanism chambers 48 and 49.

When it is desired to exert a greater force, as when the mold halves arebeginning to clamp, solenoids 53 and 55 are opened, and solenoid 51 isclosed. This then communicates chambers 42 with 48, and 43 with 49.Thereafter, continued operation of motor 25 will pump fluid betweenchambers 43, 49 and chambers 42, 48, respectively. The hydraulicconnection of the corresponding actuator chambers of the first andsecond transmission mechanisms has the effect of increasing the areaagainst which the pump pressure differential acts, to increase theclosing force imparted to the mold halves.

To unclamp the mold halves, the operation is reversed. Initially,solenoid valve 51 is closed and solenoid valves 53 and 55 are opened toallow a low-speed high-force breakaway of the mold halves. Thereafter,solenoid valve 51 may be opened, and solenoid valves 53 and 55 may beclosed, to permit a high-speed retracting movement of the output member,with the second transmission mechanism chambers 48 and 49 being incommunication with one another through conduit 50 and now-openedsolenoid valve 51.

Modifications

The present invention contemplates that many changes and modificationsmaybe made.

For example, the motor may be used to drive a threaded connectionbetween the motor output shaft and the output member. Alternatively, themotor output may be connected to a pump to provide fluid power, ratherthan mechanical power between these elements. The first transmissionmechanism may include a piston of one area operatively arranged within acylinder and used to selectively displace fluid to a second cylinder.Alternatively, the pump may selectively communicate with both pistonhead faces so as to increase the effective area of the pistons. Whilethe present invention is shown as using solenoid valves as part of thetransfer mechanism, other types of valving arrangements mightalternatively be used. The present invention uses at least one feedbacktransducer that is capable of measuring force, displacement or velocity.In the form shown, transducer 32 is employed to measure the force of theload on the output member, and transducer 29 is operatively arranged tomeasure either the velocity or the displacement of the output memberrelative to the stationary member. The reader will recall that velocityis the time derivative of position (i.e., v=ds/dt). Hence, velocity andposition are mathematically related to one another.

Therefore, while three presently preferred forms of the inventiveservoactuator have been shown and described, and various modificationsand changes thereof discussed, persons skilled in this art will readilyappreciate that various additional changes and modifications may be madewithout departing from the spirit of the invention, as defined anddifferentiated by the following claims.

1. A servoactuator for selectively controlling the movement of an outputmember in two directions in response to a command signal, comprising: anelectric motor having a rotatable output shaft; a motor controllerarranged to provide a drive current to said motor for controllablyrotating said output shaft; a first transmission mechanism operativelyarranged to displace said output member in either direction at a nominalfirst ratio with respect to said motor output shaft; a hydrostaticsecond transmission mechanism operatively arranged to displace saidoutput member in either direction at a nominal second ratio with respectto said motor output shaft; a transfer mechanism operatively arranged toselectively couple said motor output shaft to said output member eitherthrough said first transmission mechanism to impart a high-speedlow-force drive to said output member, or through said secondtransmission mechanism to impart a low-speed high-force drive to saidoutput member; at least one feedback transducer capable of measuring oneof the force, displacement or velocity of said output member, and aservo control loop closed about said motor, controller, transmissionmechanisms, transfer mechanism, feedback transducer and output memberfor selectively controlling at least one of the position, velocity orforce of said output member as a function of said command signal.
 2. Aservoactuator as set forth in claim 1, and further comprising: anengaging device for selectively coupling said second transmissionmechanism to said output member at any position of said output member.3. A servoactuator as set forth in claim 1 wherein said firsttransmission mechanism is hydrostatic.
 4. A servoactuator as set forthin claim 3 and further comprising a plurality of valve componentsoperatively arranged to couple said first and second transmissionmechanisms hydraulically when said output member is displaced at saidnominal second ratio with respect to said motor output shaft.
 5. Aservoactuator as set forth in claim 1 wherein said first transmissionmechanism includes a screw thread and nut.
 6. A servoactuator as setforth in claim 5 wherein said transfer mechanism comprises a pluralityof valves operatively arranged to selectively either (a) lock said nutto said output member and allow said second transmission mechanism to bedecoupled from said output member, or (b) couple motion of said nutrelative to said output member through said second transmissionmechanism to said output member.